I mean, for some environment where you can rely only on Emacs, we can't use `3` as it cannot be implemented in Emacs.  `1` and `2` satisfy major portion of need and can be implemented in Emacs.  But probably we shouldn't go that far anyway.  In Windows you can probably implement exec-like command in bat script.  I guess not implementing 1 and 2 until some user asks is the right direction.